Multi-layered thermoplastic article



March 7, 1967 K. J. CLEEREMAN MULTI-LAYERED THERMOPLASTIC ARTICLE 4Sheets-Sheet 1 Filed Oct. 24, 1963 INVENTOR. Kenna/6 J. C/eereman QMW MWWays K. J. CLEEREMAN MULTI-LAYERED THERMOPLASTIC ARTICLE March 7, 1967 i4 Sheets-Sheet 5 Filed Oct. 24, 1963 CONVENTION AL CONVENTIONAL ROTATEDCORE ROTATED CORE) Q CONVENTIONAL ROTATED CORE /N VE N TOR Karine/h J.C/eereman March 7, 1967 J, CLEEREMAN 3,307,726

MULTI-LAYERED THERMOPLASTIC ARTICLE Filed Oct. .24, 1963 4 Sheets-Sheet4 CONVENTIONAL ROTATED CORE SIDEWALL gm cup CONVENTIONAL ROTATED CORE IN V E N 7 0R Ken nef/v J. C/eerem'an United States Patent 3,307,726MULTI-LAYERED THERMOPLASTIC ARTICLE Kenneth J. Cleerernan, Midland,Mich., assignor to The Dow Chemical Company, Midland, Mich., acorporation of Delaware Filed Oct. 24, 1963, Ser. No. 318,745 5 Claims.(Cl. 215-1) This invention relates to a novel injection moldingtechnique, to apparatus for uniquely accomplishing said technique and toproducts produced therefrom. More particularly, this invention relatesto the injection molding of products of rotational symmetry whereinmulti-directional orientation is automatically imparted to the plasticmaterial used to mold the product.

Normally when a product is injection molded, molten plastic is injectedthrough a gate in one direction at high pressure into a relatively coldmold. The sudden chilling causes unidirectional orientation of theplastic molecules, and mechanical weakness in the transverse directionof the molded product results.

However, because injection molding is very economical for formingarticles of low-cost general purpose plastics, such as polystyrene, ithas become very popular as a plastic forming means. In the past, thetrade has disregarded or ignored many of the valuable mechanicalproperties of general purpose plastics. For example, general purposepolystyrene has a very high modulus of elasticity. It is availablecrystal clear and is easy to fabricate. It is also relatively low incost. Yet, relatively few uses of this popular plastic utilize thesehighly desirable characteristics probably because they are too difficultto attain with the forming methods currently used. If such plastic couldbe injection molded with high controlled multidirectional orientation,many new products could be made with highly useful characteristics.

Some attempts have been made to obtain multi-directional orientation,but unfortunately, they have not been successful. For example, US.2,372,177 issued March 27, 1945, claims that certain plastic materialmay be formed into relatively thin walled products with an effectivegrain direction extending helically thereof on the inner surface andlongitudinally thereof on the outer surface. This is allegedlyaccomplished by a technique wherein, during injection molding, one partis moved relative to the other by a mechanical gear train. Movement ofthe mold part is tied to movement of the injecting piston.

Unfortunately, in actual practice, articles produced using the methoddescribed, such as cups, do not possess the alleged degree ofmulti-directional strength. They still crack or splinter or break easilyalong a line either parallel to the axis of the article or in a hoopdirection. To applicants knowledge, the technique has never been adoptedby the trade, probably due to the fact that no one could successfullyproduce an article having the alleged multi-directional strength.

.One object of this invention is to provide a novel injection moldingprocess.

Another object is to provide an injection molding process whichautomatically manufactures rotational symmetric products uniquelypossessing high multi-directional strength.

A further object is to provide apparatus which is capable of automaticinjection molding of plastic products of rotational symmetry with a highmulti-directional orientation.

3,307,726 Patented Mar. 7, 1967 ice A still further object is to provideinjection molding apparatus, of the above character, which is capable ofhigh speed, economical operation.

Another object is to provide plastic products of rotational symmetryhaving high strength in all directions.

Another object is to provide molded plastic products which have a uniquelaminar structure.

Another object is to provide molded plastic products of rotationalsymmetry wherein the molecular pattern is uniquely helicoidal throughthe thickness of the wall.

Another object is to provide plastic products of rotational symmetrywhich, by reason of their multi-ply structure oriented in differentdirections, resist puncturing, cracking, chipping, tearing and the like.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the several steps and the relationof one or more of such steps with respect to each of the others, theapparatus embodying features of construction, combinations andarrangements of parts which are adapted to effect such steps, and theproduct which possesses the characteristics, properties and relation ofelements, all as exemplified in the detailed disclosure hereinafter setforth, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings, in which:

FIG. 1 is a cross-sectional side view of an embodiment of an injectionmolding machine capable of performing the novel technique to produce aunique plastic article with multi-directional strength.

FIG. 2 is a top view of an extruder for feeding plastic in fluidcondition to the apparatus of FIG. 1.

FIG. 3 is a perspective view of a cup after being subjected to adestructive force to cause cracking of the sidewall, said cup havingbeen molded on the apparatus of FIG. 1 in accordance with the noveltechnique discussed in detail hereinafter.

FIG. 4 is a cross-sectional view of the cracked wall of the cup of FIG.3 taken along lines 44.

FIG. 5 illustrates the birefringence profile through the sidewall andbottom of a cup from the direction of flow in accordance with thecustomary molded prior art technique as compared to a cup molded inaccordance with the technique of the invention.

FIG. 6 illustrates a similar comparison between the birefringenceprofile about a point on the outside surface of the bottom of a cup 90from the direction of flow.

FIG. 7 illustrates a similar comparison of the birefringence profileabout a point on the outside surface of the side wall of a cup 90 fromthe direction of flow.

FIG. 8 illustrates the birefringence anisotropy, as viewed from theoutside in, of a center plane of the bottom of two cups or tumblers, theupper of a cup molded in accordance with the prior art technique, andthe lower of a cup molded in accordance with the technique of thisinvention.

FIG. 9 illustrates a similar comparison of the birefringence anisotropyof the center plane of the sidewall of said cups.

FIG. 10 is a composite orientation pattern of four parallel planarsurfaces through the bottom of two cups or tumblers, the upper of a cupmolded in accordance with the prior art techniques and the lower of acup molded in accordance with the techniques of this invention.

FIG. 11 illustrates a similar comparison of composite orientationpatterns through the sidewall of said cups or tumblers.

Similar reference characters refer to similar parts throughout theseveral views of the drawings.

It has now been found that plastic products of rotational symmetry withhigh multi-directional orientation can be formed by the economicalinjection molding technique if one of the mold elements is rotated witha level of torque at least equal to the static torque level of the moldequipment and the particular plastic raw material being used so thattorque will beapplied not only while the liquid plastic is beinginjected into the mold cavity, but also while it is solidifying to itssecond order transition point, With such an arrangement, a point will bereached during the molding cycle whereby the plastic will acquiresufiicient resistance to the applied torque that it Will cause therotating mold element to stall. Thus there is application of torqueuntil the orientation imparted to the molecules is frozen in.

More particularly, it has been found that when hollow plastic products,all or a portion of which have rotational symmetry, are injection moldedwhile one of the mold elernents forming said rotational symmetricportion is r tated with a torque of a level which permits continuedrotation after the mold cavity is filled but is insufficient to overcomethe resistant forces generated by the thermoplastic when it reaches itssecond order transition point (the point of solidification), themolecules therein will attain a high degree of multi-directionalorientation in a layer-like helicoidal pattern of molecular thicknessthrough the wall of said article. At such level of torque, theorientation is frozen into the article so that it will thereafterpossess multi-directional strength. To define said torque level inanother way, the torque should be sufficient to cause continued rotationof the rotatable ele ment for short time after the mold is filled, butinsufiicient to damage or prevent solidification of the molded article.

The rotation of the mold element is preferably accomplished, as seen inFIG. 1, by rotating one member of the mold with a rotational means,wherein a certain preset level of torque is applied to the plastic as itflows into the mold cavity and solidifies. The level of torque applieddepends upon the raw thermoplastic material used, the shape beingmolded, the surface characteristics of the mold, the machine utilized,and similar factors. Optimally, the level should be that of statictorque which may be defined as the minimum torque needed to lock orstall the rotating mold element when the thermoplastic reaches itssecond order transition point.

One preferred torque applying means for the rotatable element of themold is a presettable constant torque motor. It is capable of rotatingthe male member until the cooling thermoplastic article sets upsuificient resistant force to cause stalling. However, the presenttorque of said motor continues to act upon the mold element until theplastic solidifies. At this stage, the plastic is at its second ordertransition point.

The continued application of torques freezes in the orientation whichwas imparted to the molecules of said plastic during its injection intothe mold cavity. Only 'at this time is the application of the actuatingforce terminated. The mold is now opened and the plastic articleremoved.

In essence, the process causes orientation by creating a conditionwherein molecular flow is retarded while an elastic strain is placedupon the molecules. By freezing in this elastic strain due to cooling ofthe plastic, an oriented structure is uniquely achieved. Heretofore,such freezing in of elastic strain during cooling has been over lookedand this is probably the reason prior art teachingswere inoperable.

The article molded as described above possesses a high degree ofmulti-directional orientation and has good flexibility in alldirections, good crush strength, and is puncture resistant.

Examination of samples reveals the orientation is not biaxial. It ismore fan-shape throughout the thickness of the article. At the midpointof the cross-sectional thickness, the orientation is mainly in the hoopdirection; at one surface, the orientation is about while at the othersurface it is about -45. A very strong semilaminated structure isevident and such lamination obviously greatly increases the toughness ofthe molded article. When a destructive force is applied, amulti-laminated breaking occurs; and it becomes readily evident that thearticle has a plural ply structure.

Reference is now made to the drawings for a detailed description of theinjection molding technique of the apparatus for uniquely effecting saidprocess, and of the article that may be made therefrom.

As seen in FIGS. 1 and 2, which illustrate one embodiment of apparatusfor effecting the novel injection molding technique to produce theunique article of this invention, the apparatus has a nozzle valve 10and an injection plunger 12, both of which individually reciprocatewithin injection cylinder 14 to supply a metered amount of fluidthermoplastic material to the mold cavity defined by an outer stationarymold element 16 and an inner mating rotatable mold element 18.

A large hydraulic accumulator (not shown) is preferably used on theforward stroke of the injection plunger 12 to obtain high injectionspeeds. The injection valve prevents drooling and permitsprepressurizing of the plastic in the injection cylinder, before it isreleased to the mold cavity. Indeed, it is preferred that this valve beactuated by a second hydraulic system which also utilizes an accumulator(not shown) so that in operation, both the valve accumulator and theplunger accumulator may be precharged to effect a desirable high speedand high pressure operation to the apparatus. Thus, both valves may besimultaneously opened after the mold cavity is closed to provide acontrollable and extremely rapid injection speed. Control of this speedalong with control over the temperature of the plastic and theextrusion, as will be explained hereinafter, allows considerable controlover the orientation in the longitudinal direction. Control overorientation in other directions is effected by the unique operation ofthe rotatable mold element 18.

In the embodiment shown, molten, or more accurately, fluid thermoplasticis fed to the injection cylinder 14, in continuous fashion by extruder22 (see FIG. 1) via flex ible feed line 24, containing an elbow 26 forflexibility.

A screw preplasticizer can also be used.

To effect accurate control over the pressure of extrusion, feed line 24contains a relief valve 28- and a pressure control 3-0 which regulatesmovement of the ram of the extruder. Raw plastic, usually in bead orpellet form, is fed to the extruder via hopper 32, the top of which isseen in FIG. 2. Metering is obtained by presetting the stroke of theinjection plunger 12. Now when the injection cylinder 14 is full whilethe plunger is still retracted, excess plastic is vented out throughrelief valve 28. Check valve 52 at the entrance to the injectioncylinder 14 prevents the metered amount of polymer within the cylinderfrom escaping out the vent during the injection stroke of the plunger.

As indicated above, orientation of the plastic molecules in otherdirections is effected by the apparatus herein in that one of its moldelements is rotatable and there are controls over the torque applied tosaid element. In this instance, the male element of the mold cavity isrotatable; however, it should be understood that the female elementcould likewise be made rotatable.

As shown, the male element 18 comprises the forward end 40 of a mandrel42, which is rotatably secured within a thrust bearing 44 within block46. The entire block reciprocates upon slide rods 47 to permit openingand closing of the mold cavity.

Mandrel 42 and likewise the male element 18 is rotated by means of apresettable constant torque motor 48 via gear assembly 50 on the rear orother end of the mandrel 42.

Since it is presettable, motor 48 is capable of generating a torque upto a certain level. It should be evident that most any constant torquemotor could be used. A hydraulically operated motor, such as a Vickersconstant displacement piston motor No. MF2012-30-61, is a noteworthyexample. With such a motor, oil under pressure, generated by a 25horsepower Vickers pump capable of delivering seventeen gallons perminute of oil at 2000 p.s.i. is preferably used to supply the hydraulicpower. Torque is controlled with a pressure relief valve (not shown) andthe speed is controlled with a flow control valve (not shown). As anexample of the level of torque cabable of being applied with a gearreduction of six-toone, the motor, driven at a 1500 p.s.i. level, isable to apply a torque of 5400 lbs. per inch on the male element.

Another very useful mot-or is an electrical direct current series roundmotor such as the Model Super T sold by Reliance Electric andEngineering Company. Reference is now made to a specific example whichillustrates the operation of the apparatus and the technique employed:

EXAMPLE I Ten-ounce tumblers were molded using a one and one half inchextruder, the inside diameters of which were 2.6 inches, the height 3.71inches and the wall thickness .045 inch. Styron 666 crystal, apolystyrene lpellet grade manufactured by Dow Chemical Company, wasutilized as the plastic raw material. The raw material was fed to theextruder,22, melted, pressurized, and fed to the injection cylinder 14via feed line 24.

Hydraulic motor 48, powered as described above, was turned on as themold cavity was closing. Thus, as the plastic was injected into thecavity, the torque built up to a preset pressure, as indicated in thetabular data below, at which time the motor stalled. The injectedplastic was allowed to cool while the torque continued to act upon themale element 18 and consequently the injection molded tumbler. Justbefore the mold was opened, the hydraulic pressure was turned 011 sothat the torque dropped to a zero level. In summation, the followingoperating conditions were used:

TABLE I Injection cylinder Mold temperature Clamp force Injectionpressure Sample 1A through 1L of Tables II and III9300 p.s.i.; Sample 1Mthrough 1X of Tables H and HI-- 10000 p.s.i. Torque on hydraulic motor-Varied as indicated in Table II. R.p.m. of mold Varied as indicated inTable II. Time to fill mold Approx. 1 sec. Time from start of injectionto motor stall .14 to 2.5 secs.

It might be noted that the friction of the rotating mandrel did notalways stay constant. However, to enable a report in actual torqueapplied to the plastic, a pressure transducer was connected to a highspeed recorder. With this arrangement, actual AP (oil pressure onhydraulic motor at stall minus the -oil pressure required to turn themandrel with the mold open) could be obtained and this is shown in TableII. This arrangement also permitted the plotting of a curve of oilpressure vs time which gave a measure of the time the motor turned afterplastic started to fill the mold. From this the seconds required tostall the hydraulic motor was determined.

FIGS. 3 and 4 illustrate a tumbler molded in accordance with the exampleabove. In the illustration, an attempt has been made to define themulti-ply structure of the tumbler by reference to a crack 62 in thesidewall. The crack 62 propagates in a random direction and diagonallythrough the thickness of the sidewall. The crack shown, has, in thisinstance, attempted to advance along a left-handed helix at the innerface, in the hoop direction in the middle, and along a right-hand helixat the outer face of the sidewall. It is a wandering compromise over theusual straight line parallel to the axis of a tumbler molded with noorientation.

TABLE II.BURSTING PRESSURE OF TUMBLERS WITH DIFFERENT DEGREES 0FMULTIAXIAL ORIENTATION Hydraulic Torque, lb. 111., Bursting Secs. reqdto AP 011 Sample Mold, pressure applied by Rpm. of pressure, p.s.i., musing stall hydraulic hydraulic Corrected N o. temp. on motor hydraulicmold using water water motor motor torque, lb.-in.

mo or As stated, FIG. 4 illustrates the diagonal crack across thethickness of the sidewall following somewhat the multiplanes .therein.

It might also be noted that the crack has a large area per unit length.In tumblers without orientation, the crack,'being parallel to the axisof rotation, vertically linear and radially in a straight line throughthe sidewall, has minute area per unit length.

Insofar as visual appearance goes, it is not possible to tell anydifference between a conventional injection molded tumbler and one inwhich the male half. of the mold was rotated. Such two tumblers weresubjected to various tests as follows:

Puncture testing Table II shows the results of these tests. As can beseen from the table, as the torque applied by the hydraulic motor on thetumbler increased, its bursting strength increased. It appears that amaximum strength is reached after which the bursting pressure holdsconstant with increasing degrees of orientation or it may even decrease.

It might be noted that the bursting strength of tumblers with highmulti-axial orientation is double that of turnblers with no multi-axialorientation.

The type of fracture obtained when testing in the burst tester was alsostudied. The tumblers with the lesser de grees of multi-axialorientation and burst strength give a much more brittle failure.

Solvent burst strength test Samples 1A, 1C, 1E, 1F, II, and 1K were putin the burst tester with Mazola oil as the pressurizing fluid. The hoopstress was held at 2000 psi. and the time required for the tumblers tofail was measured. Data is shown in Table III.

As can be seen from the data, as the orientation increased, the timerequired to fail increased. Tumblers molded without rotating the maleelement broke in 0.1 minute; however, tumblers with a high degree ofmultiaxial orientation went several thousand minutes before failure.

TABLE III.MECHANICAL PROPERTIES OF TUMBLERS WITH DIFFERENT AMOUNTS OFMULTIAXIAL ORIENTATION Flexure Strength (Hoop Direction) Level ofmultiaxial Time to burst orientation as minutes at Heat DistortionSample N 0. measured by the a =2,000 p.s.i., (V icat Test) 0. Testcondition Test condition Tensile Strength torque applied by in Mazolaoil b(1)], [b(2)], Long. Dir. Test the hydraulic motor to break to breakcondition [0],

p.s.i. to break *Did not break, crazed after approx. 3,600 minutes-at4,320 minutes hoop stress increased to 6,500 p.s.i. before tumblerbroke.

Squeeze test A really dramatic test was slamming both tumblers against asurface and on the floor so that each tumbler would hit on the bottomradius. The tumbler without rotation always flew apart. The tumbler withrotation would bounce back with only a laminar fracture at the contactpoint.

N on-so lvent burst test A burst tester was designed to test differencesbetween uniaxially oriented tumblers.

In such testing, the bottom is first out 01f the tumbler and then it issealed at the top and bottom in the tester and filled with water. Thepressure is then increased until the sidewall breaks. The pressure tobreak is recorded from which the hoop stress is calculated using theequation:

r:1.305 inches b=.045 inch P=p.s.i. to burst M echunical tests (a) Pointpressure.2" X 2" sections were cut from the wall of a tumbler. Thesewere then set in a jig and tested as a diaphragm with center pointloading. The test data on this type test was meaningless. Samplescracked but no realistic failure point could be determined. However,when examining samples removed from the tester before final failure,interesting crack patterns were observed. Sample A with Zero hooporientation broke as a straight line parallel to the flow direction,Sample E with a moderate amount of hoop orientation crackedat an angleto the flow direction while Sample I with high hoop orientation brokewith the typical laminar break of a multi-axial oriented sample.

(b) Flexure.Rings were cut from the side wall of the tumbler and testedin flexure. Flexure tests were run with a 16-1 span to thickness ratio.Samples were /2 inch wide X .045 thick and were tested in two ways.

(1) With the concave side against the two fixed supports.

(2) With the convex side against the fixed supports.

Table III shows the results of these tests. As can be seen, the strengthof the samples tested in flexure increases as the degree of hooporientation increases. The percentage increase indicates that a veryhigh degree of hoop orientation has been obtained.

(c) Tensile strength.Samples were cut from the sidewall of the tumblersin the longitudinal direction. These samples were then milled into atypical tensile shape and tensile data obtained.

The cross section at the center was .21" x .045". Curved grips were usedin the tensile tester to avoid cracking in the grips. Data are shown inTable III.

As can be seen, as the degree of orientation increases, the tensilestrength in the flow direction uniquely de- Such reduction in tensilestrength may be explained as follows:

When filling a mold, orientation depends upon the shear stress appliedto the plastic as it flows. This shear stress depends upon the polymerviscosity, temperature of the plastic and mold, and the rate of fill. Byrotating one element of the mold, the shear stress required to fill themold decreases.

Heat-shrink testing When a tumbler molded with zero rotation is placedin an oven at 115 C., it shrinks in both length and radius. At the sametemperature, a tumbler molded with moderate rotation (1800 lbs.-in.torque) warpsbecause stresses are developed due to the fact thatdifferent layers of polymer are attempting to shrink in difierentdirections. Vertical cuts made prior to the oven treatment eliminatesome of this mutual restraint, and the tumbler walls twist somewhat.

A narrow vertical strip from a zero-rotation tumbler shortens in theoven whereas a similar strip cut from a tumbler molded with rotationtwists into a helical form when heated. Finally, a semicircular samplecut from the bottom of a zero-rotation cut shrinks in the radialdirection, and opens up (like a fan) in the zero-direction,

- while a similar sample cut from a cup molded with rotation curls up aswell. This shrinkage behavior is consistent with the postulated patternof frozen-in orientation.

Birefringence The orientation of plastic may be accomplished by theproper adaptation of mechanical contrivances to stretch, draw, roll, orextrude the material at temperatures above the second order transitiontemperature. The orientation retained by a material produces an opticalanisotropy or birefringence. The birefringence may be used as an indexof the amount of orientation.

Procedure vOne method for determining the average birefringence of abiaxial oriented sheet is the wedge method. It allows the measurement ofbirefringence at each of the two surfaces and through the thickness ofan oriented sheet. This method of measuring birefringence serves tofinger print, to identify, or to characterize the orientation pattern,and is particularly suited for thick film or sheet. It has been usedquite extensively in the orientation studies of heavy gauge biaxiallyoriented polystyrene sheet. This same wedge method was used here tostudy the orientation patterns of the injection molded tumbler.

In all measurements, two types of tumblers were studied. One was theconventionally molded tumbler in which the principal axis of orientationwas in the flow direction of the melt which is from the sprue in thecenter of the bottom of the tumbler, outward radially across the bottomand then in a straight line toward the lip, parallel to the centerlineof the tumbler. The other was the molded tumbler made with the male moldelement or core being rotated.

Within each tumbler two localized areas were selected for study. Onepoint 64 (see FIG. 3), was selected in the bottom of the cup from thecenter 66 where the gate for the mold cavity was located. Theorientation at this point in the cup made with the rotated core would besimilar to orientation induced when a plastic is sheared between twoflat circular discs which have a dilference in rotational velocity. Thesecond point 66 selected for study was located in the sidewall of thecup 1%" from the bottom. The orientation in this case, where the core isrotated, would be similar to orientation induced when a polymer issheared in the space between rotating cylinders. In either case,however, axial motion is involved.

For the conventional cup the orientation in both the bottom and sidewallcould be representative of orientation induced by the shearing action ofpolymer flowing between parallel plates.

At each of these two points, the orientation was actually measured infour directions in the plane of the shell. For the bottom of the cup,the directions indicated by the arrows were (1) the radial flowdirection arrow 68, (2) 45 clockwise from flow arrow 70, (3) from flowarrow 72, and (4) 45 counterclockwise from flow arrow 74. In thesidewall of the cup, the directions were (1) the axial flow directionarrow 76, (2) 45 clockwise from flow arrow 78, (3) 90 from flow which iscalled the hoop direction arrow 80, and (4) 45 counterclockwise fromflow arrow 82. An infinite number of directions could have been used,but four directions appeared suf ficient to establish the generalpattern of orientation characteristics:

(a) Birefringence profile photograph. Looking edgewise at wedges cutwith their bases inthe 90 direction from flow, the birefringenceprofiles for the bottom and sidewall of tumblers made without and with atorque generating mold element were photographed. (See FIG. 5). Thefringe patterns of these photographs is the orientation observed whenlooking at the edge of a section cut from the shell of the cup; thecloser together the fringes, the higher the orientation, and the higherthe strength. It will be noted that without a torque generating moldelement,.little, if any, orientation is attained at the point 90 fromflow. Thus there will be no hoop strength.

'shown as high point on the curve.

The birefringence profile for the bottom of the conventionally moldedtumbler (top curve of FIG. 6), indicates that there is no appreciableamount of orientation exhibited anywhere through the wall thickness.Consequently, if pressure were applied to the bottom of the cup, itwould split easily in a straight line from the center of the bottomoutward radially to the wall of the cup. Now looking at the bottom ofthe cup made with the rotated core (FIG. 6-bottom curve), it is seenthat the orientation has been increased significantly. The skin orsurface orientation in the flow direction where the polymer contactedthe mold and core remains essentially the same as for the non-rotatedcore. But the orientation is almost immediately high through thethickness of the shell. It is thus seen that by rotating the core, asignificantly tougher bottom has been made. Any break therein wouldassume a random pattern trying to seek the weakest direction in eachlayer.

FIG. 7 shows the birefringence profiles in the sidewall. The differencesbetween the upper and lower curves noted are analogous to thedifferences discussed above concerning the bottom. And while the netamount of orientation is apparently lower for the sidewall, it is stillsufficient to provide high strength.

(c) Birefringence anis0iropy.VieWing the cup from the outside in, theorientation pattern shown in FIG. 8 of the center plane exhibits noorientation for the conventional tumbler or cup. It would be brittle andwould add nothing to the strength of the cup. In contrast, the cup madewith a torque applying mold element, shows a center layer with anappreciable amount of orientation. Since it has significantly moreorientation, it obviously exhibits greater toughness than theconventional cup.

FIG. 9 shows a similar orientation pattern for the center layer of thesidewall. The conventional cup again has no orientation while the centerlayer of the rotated core cup exhibits appreciable orientation in thehoop direction.

((1) Composite orientation curves.FIGS. 10 and 11 show, respectively,orientation patterns for five layers through the bottom and sidewall ofthe tumbler 'orcup which have been superimposed. Such patterns areanalogous to a five layered piece of laminated plywood. For theconventional cup, it is evident that four of the layers have their grainrunning in the same direction while the center layer has no grain atall. Such structure is easily split or cracked. The cup made by rotatingthe core has layers wherein the grain structure runs in differentdirections. The net effect is a significantly tougher part.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efiiciently attained and,since certain changes may be made in carrying out the above process, inthe described product, and in the constructions set forth withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description (or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention, which, as amatter of language, might be said to fall therebetween.

Now that the invention has been described, what I claim as new anddesire to secure by Letters Patent is:

1. A thermoplastic article having rotational symmetry and amulti-layered structure, each layer of such structure having a molecularorientation different than the orientation in an adjacent layer thereto,whereby multidirectional strength is imparted to said article comprisingan injection molded plastic with a birefringence pattern through itsthickness at 90 from flow, substantially similar to that shown in thepat-terns effected with rotation in FIG. of the drawing herein.

2. A thermoplastic article having rotational symmetry and amulti-layered structure, each layer of such structure having a molecularorientation different than the orientation in an adjacent layer thereto,whereby multi-directional strength is imparted to said article,comprising an injection molded structure, which has a birefringenceprofile pattern about a point of 90 from flow which exhibits an almostimmediate high orientation through the thickness of the plasticstructure.

3. A thermoplastic article having rotational symmetry and amulti-layered structure, each layer of such structure having a molecularorientation different than the orientation in an adjacent layer thereto,whereby multidirectional strength is imparted to saidarticle,'comprising an injection molded structure, wherein the area ofthe birefringence anisotropy curve of the central layer of the structurehas an area at least two times the area of the birefringence anisotropycurve of the central layer of a similar structure without orientation.

4. A thermoplastic article having rotational symmetry and amulti-layered structure, each layer of such structure having a molecularorientation different than the orientation in an adjacent layer thereto,whereby multi-direction- 'al strength is imparted to said article,comprising an injection molded structure, wherein the superimposedcomposite orientation curve for a plurality of layers spacedequidistantly across the thickness of the structure of said articleevidences when overstressed a grain structure which runs in differentdirections than the direction of flow.

5. An injection molded thermoplastic article having rotational symmetrycomprising a multi-layered structure greater than three, each layer ofsuch structure having a molecular orientation different than theorientation in an adjacent layer thereto whereby multi-directionalstrength is imparted to said article, the center layer havingappreciable orientation in the hoop direction of said article.

References Cited by the Examiner JOSEPH R. LECLAIR, Primary Examiner.ROBERT F. WHITE, Examiner. R. B. MOFFITT, D. F. NORTON, Assistant Exminers.

1. A THERMOPLASTIC ARTICLE HAVING ROTATIONAL SYMMETRY AND AMULTI-LAYERED STRUCTURE, EACH LAYER OF SUCH STRUCTURE HAVING A MOLECULARORIENTATION DIFFERENT THAN THE OREITNTATION IN AN ADJACENT LAYERTHERETO, WHEREBY MULTIDIRECTIONAL STRENGTH IS IMPARTED TO SAID ARTICLECOMPRISING AN INJECTION MOLDED PLASTIC WITH A BIREFRINGENCE PATTERNTHROUGH ITS THICKNESS AT 90* FROM FLOW, SUBSTANTIALLY SIMILAR TO THATSHOWN IN THE PATTERNS EFFECTED WITH ROTATION IN FIG. 5 OF THE DRAWINGHEREIN.